With their tertiary structures often collapsing (denaturing) under non-physiological conditions, proteins and peptides sometimes give rise to precipitations that are insoluble in water. For instance, the molecular mechanism of aggregations of a protein is thought to be one whereby hydrophobic amino acid residues, which are buried inside the molecule for a protein in the native structure, become exposed with denaturation and, with the intermolecular hydrophobic interactions becoming stronger, aggregation is enhanced (refer to FIG. 1, left). As one approach to confer high solubility to a protein with low solubility in water such as a protein in a denatured state, a technique has been developed, whereby a functional group with high hydrophilicity is introduced using chemical modification methods. As this functional group with high hydrophilicity, one processing an electric charge is suitable, and in particular a functional group carrying a positive charge (cation) is advantageous (refer to FIG. 1 right and Non-Patent Literature 1).
At right of FIG. 1, (a) is an example in which an irreversible cationization reagent has been used, in this case, reconstitution (refolding) after solubilization is not possible. In contrast, if a “reversible (denaturing) cationization” technique, which confers a positive charge through a reversible disulfide bond (SS bond), is used on the Cys (cysteine) residues inside the protein, as inside the dotted line at right of FIG. 1 ((b), (c)), it is also possible to dissociate the reagent used in the cationization with a reducing agent as necessary (refer to Non-Patent Literature 2).
As reagents for cationizing a protein reversibly, for instance, TAPS-sulfonate (trimethylammoniopropyl methanethiosulfonate bromide) has been developed and commercialized (refer to Non-Patent Literatures 3 and 4). This reagent can add a univalent quaternary ammonium ion to a protein in denatured state through an SS bond. In addition, derivatives of a polymer having a cationic group (cationic polymer) such as polyethyleneimine (PEI) have been described (refer for instance to Patent Literatures 1 to 3). As PEI derivatives, for instance, PEI-SPDP (mixed reaction reagent of polyethyleneimine and N-succinimidyl-3-(2-pyridylthio)propionate), and the like, are described in Patent Literatures 1 and 2.